A need exists for an inexpensive, lightweight, tunable laser radiating in the far infrared wavelengths. There are many gas lasers available in the far infrared wavelengths; however, they are not tunable. Free electron lasers are tunable, but they are expensive and bulky.
It has been suggested that stimulated emission by relativistic electrons gyrating in a magnetic field might be possible if there were an overpopulation of electrons in the upper energy states. See J. Schneider, Stimulated Emission of Radiation by Relativistic Electrons in a Magnetic Field, Phys. Rev. Lett. 2, 504 (1959). A gyrotron in which electrons gyrate in a vacuum would be tunable, but wavelengths produced would be much longer than infrared wavelengths.
B. Lax, in "Cyclotron Resonance and Impurity Levels in Semiconductors," (Quantum Electronics, edited by Charles H. Townes, Columbia University Press (New York, 1960), p. 428, proposed to use indium antimonide, InSb, which has a nonparabolic conduction band and small effective electron mass to produce coherent radiation. However, his idea of making a population inversion by optical pumping is apparently inoperable because the lifetime of electrons in the Land levels is too short.
Ganguly, et al., in U.S. Pat. No. 4,376,917, have suggested a device for injecting electrons in indium antimonide to make a cyclotron maser. Although the Ganguly device may create hot electrons, it apparently does not provide for an electron population inversion.
Heiblum, et al., have recently presented the first direct evidence of ballistic electrons in a thin gallium arsenide, GaAs, layer disclosing an overpopulation of electrons in the upper energy states. See (Heiblum, et al., "Direct" Observation of Ballistic Transport in "GaAs", Phys. Rev. Lett. 55, 2200 (1985)).